Struggeling with Image Quality Comparison

Dear Community,

i just started using Premiere pro very recently and i am already a huge Fan of it and would like to convince my boss to use it.

However, i fail to get the same Image/Audio Quality/storage consumption ratio from Premiere Pro.

Currently we rely on a >20 year old video editing solution which has "Windows XP" as recommended operating system. And its not just a "One programm" solution. We are using 6 different tools in our default workflow. However - this toolset still performs pretty well to be honest.

The only downsides are: it doesnt support hardware acceleration what-so-ever which isnt too suprising. And we have to jump continously from one tool to another. Which isnt really comfortable.

We are mainly doing content in 1080p,25fps,H265,Main,4.0.

And we are getting rendering speeds of ~80-90 fps (CPU only).

I tried rendering the same material with roughly the same settings in Premiere Pro and ended up with ~185 fps (CPU+GPU). Which was very promising. Unfortunately: the Image Quality couldnt keep up to our standards.

And in addition: the audio Quality couldnt as well. Since most of our content is in stereo and basically just speech we ended up using m4a Dolby Pulse 44khz 64 Kbit/s - which fits to our demands really nicely.

To summarize:
I need to improve image quality and audio quality but i must keep the storage consumption low. Rendering speed is secondary and not really my problem.

My question is (and please keep in mind that i am very new to Premiere Pro):
Which settings shall i try to improve the quality without increasing the filesize?

And another thing i am also wondering about: does using NVENC still decrease the image quality? I thought this is not an issue anymore?

Thanks and BR,

Henning